It will be a six-to-eight week absence for Amar’e Stoudemire, a development that has Carmelo Anthony astonished.

“It’s shocking,” Anthony said yesterday.

Anthony’s star sidekick will undergo left knee surgery this week, sidelining him until at least mid-December and possibly for the rest of the calendar year. The Knicks say the 29-year-old Stoudemire will have a debridement, which is a cleaning out of loose bodies. It’s a far worse timetable than originally set. Stoudemire had been diagnosed with a ruptured cyst behind the knee and was expected to be out only one or two more weeks.

Instead, Stoudemire — entering the third season of his five-year, $100 million pact — is out for potentially four times as long as anticipated. Six weeks from yesterday would put him at Dec. 11 (the season’s 21st game) and eight weeks would put him at Dec. 25 (the 28th game).

It would be reasonable for Stoudemire to possibly make his return for the Christmas clash vs. the Lakers. But even that may be optimistic. Who knows how he’ll come through the operation (he also had microfracture surgery on the knee in 2005) or how his rehab will proceed?

What’s more, Stoudemire will also have conditioning issues because he won’t have played all season. It’s certainly feasible he’ll need more time than the eight weeks and won’t return until January 2013.

“At first it was two-to-three [weeks]. So we were looking forward to having him back still early,” Anthony said. “Just to hear that, out six-to-eight now, obviously the injury was very severe, more severe than what we thought it was gonna be.”

The news is plenty damaging to the Knicks, who have no ideal backup power forward and who also will have to re-incorporate Stoudemire back into the offensive and defensive mix when he does return. This will disrupt Mike Woodson’s in-season rotation and will only further strain the chemistry issues between Stoudemire and Anthony.

“I definitely thought this was gonna be the season that we’d have a chance to go out there and play together in a full season, no injuries and things like that,” Anthony said. “But at the end of the day, you can’t control that.”

Added Tyson Chandler, “[Amar’e’s] been one of the hardest workers throughout the entire summer. Been motivating, keeping the team on track, sending out mass text messages. I feel more for him personally. But like I told him, it’s a little setback in the big picture.”

Stoudemire missed the first two preseason games with what was termed a “bruised left knee.” He then played 27 minutes on Oct. 19 against the Raptors, scoring 18 points. But two days after that, it was announced he had the cyst.

Without Stoudemire, the Knicks have a major scoring void behind Anthony. Two of last season’s top three scorers (Stoudemire and Jeremy Lin) and three of the top six (Iman Shumpert being the other) will not be on the floor to start the season. Last year the Knicks averaged more points without Stoudemire (99.1 in 19 games) than with him (97.4 in 47 games).

Woodson said he’ll start his power forward “based on personnel, who other teams start.” Kurt Thomas is a possibility, and Woodson mentioned Rasheed Wallace could play there and moving Anthony to the four.

“It’s tough. Amar’e’s a big piece to our puzzle. I don’t care how you slice it,” Woodson said. “We’ll be here when he gets back.”